Regions
Northwest Coastal.
As snow accumulates on Monday, the Avalanche Danger will rise. Expect reactive new storm slabs to form.
Confidence
Fair - Intensity of incoming weather systems is uncertain on Monday
Weather Forecast
Another pacific frontal system will bring snowfall to the coast on Monday. Lingering flurries are expected on Tuesday before a weak dry ridge of high pressure builds on Wednesday. Sunday night and Monday: Up to 20cm of snow / Strong southwest winds / Freezing level at valley bottom Tuesday: Very light snowfall and broken cloud / Moderate southwest winds / Freezing level at valley bottom Wednesday: Broken cloud / Moderate southwest winds / Freezing level at valley bottom
Avalanche Summary
Several storm slabs to size 2.5 were reported in the wake of Thursday night's storm. With more snow and wind on the way on Sunday, I'd expect continued storm slab activity. It's important to keep in mind, a surface avalanche in motion may be what it takes to trigger deeper, more destructive persistent slab avalanches.
Snowpack Summary
Prior to the weekend, moderate to heavy snowfall and strong southwest winds built deep storm slabs which are expected to be most reactive in wind-exposed terrain. More snow and wind forecast for Sunday night and Monday will add to this ongoing storm slab problem.About 100cm below the surface, you'll likely find a rain crust which was buried on January 26. Limited observations suggest the overlying slab may have a reasonable bond at this interface.A rain crust and/or surface hoar layer buried mid-January may be found at variable depths (in some areas up to 150cm below the surface). This layer is likely gaining strength, although I would keep it on my radar especially at higher elevations as it was the culprit in much of the recent destructive avalanche activity.The November crust/ facet combo near the bottom of the snowpack is thought to be generally well bonded. However, this deep and destructive layer is still reported to be reactive in the far north of the region.
Problems
Storm Slabs
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
Persistent Slabs
Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.